Summary
Addressing the contradiction in the bilayer nickelate La3Ni2O7 between a bulk nodeless energy gap and d-wave-like features in point-contact transport, the paper theoretically assumes that interlayer pairing driven by strong interlayer antiferromagnetic coupling is the dominant pairing channel and introduces interlayer and intralayer pairing amplitudes in a two-orbital tight-binding model to reproduce the multi-gap structure seen by ARPES/STM. It is pointed out that when point-contact coupling to the two layers is inhomogeneous, a contact potential that is odd under layer-exchange mirror symmetry can act as an effective Zeeman field on the layer pseudospin, thereby binding in-gap Andreev states in the barrier region that shift with barrier width and strength; in the mirror eigenbasis, this barrier mixes mirror sectors with opposite pairing sign and opens a sign-reversal Andreev channel. Simulations for a top-layer barrier on La3Ni2O7 show that its subgap conductance can reproduce the orientation-dependent zero-bias conductance peaks and side peaks usually taken as a d-wave fingerprint, indicating that a nodeless interlayer pairing state can mimic d-wave Andreev signals and providing a natural picture for reconciling conflicting experimental evidence.
Materials
Methods
- Bogoliubov-de Gennes (BdG) Hamiltonian calculations
- two-orbital tight-binding model
- Blonder-Tinkham-Klapwijk (BTK) formalism
- multichannel BTK conductance calculations
- open-slab calculations
- wave-function matching
Keywords
- interlayer pairing
- d wave pairing
- andreev bound states
- zero bias conductance peak (zbcp)
- mirror odd barrier
- layer pseudospin
- effective zeeman field
- mirror selective andreev response
- sign reversal pairing
- nodeless gap
- point contact andreev reflection spectroscopy
- orientation dependent conductance
Highlights
- The layer degree of freedom acts as a pseudospin, allowing a nonmagnetic mirror-odd potential to behave as an effective pseudospin Zeeman field.
- Interlayer pairing produces symmetry-enforced opposite-sign gaps in the two mirror sectors residing on closely spaced Fermi surfaces, unlike canonical s± pairing in iron-based superconductors.
- The d-wave-like ZBCP should be tunable through the layer asymmetry of the contact and disappear in the mirror-even limit of equal coupling to the two layers.
- A mirror-even barrier yields a two-gap s-wave conductance spectrum, while a mirror-odd barrier produces a ZBCP along one direction and side dips.
- Uncontrolled variations in barrier parity and junction orientation may account for differences among existing point-contact experiments.
Conclusions
- A nodeless interlayer pairing can naturally mimic d-wave Andreev signatures when the point contact is not evenly coupled to the two layers.
- A contact potential odd under layer-exchange mirror symmetry acts like an effective Zeeman field on the layer pseudospin, binding in-gap Andreev states that sweep through the gap with barrier width and strength.
- In a realistic model for La3Ni2O7, the simulated subgap conductance for a top-layer barrier reproduces the orientation-dependent ZBCP usually interpreted as a fingerprint of d-wave pairing.
- Interlayer pairing can reconcile a nodeless bulk gap with strongly contact-dependent and orientation-dependent conductance spectra in high-Tc bilayer nickelates.
- A ZBCP in point-contact spectroscopy is not by itself definitive evidence for d-wave pairing.
Main claims
- A nodeless interlayer pairing state can mimic d-wave Andreev signatures when the point contact couples unevenly to the two layers.
- Evidence: abstract statement that nodeless interlayer pairing mimics d-wave Andreev signatures for uneven point-contact coupling,minimal-model derivation showing mirror-odd potential acts as effective Zeeman field,conductance calculations showing ZBCP for mirror-odd but not mirror-even barriers
- A contact potential odd under layer-exchange mirror symmetry acts as an effective Zeeman field on the layer pseudospin and binds in-gap Andreev states.
- Evidence: block-diagonal partner-frame Hamiltonian where mirror-odd potential enters with the same sign for electron and hole,round-trip phase condition giving bound-state energy dependent on barrier width and strength,numerical bound-state energies reaching zero in several parameter regimes
- In a realistic La3Ni2O7 model, a top-layer barrier reproduces the orientation-dependent ZBCP and side dips usually taken as a d-wave fingerprint.
- Evidence: open-slab calculations showing sub-gap Andreev branch approaching zero inside the coexistence window,multichannel BTK conductance showing broad ZBCP along x and zero-bias dip along y,side dips below normal-state conductance along x
- A ZBCP in point-contact spectroscopy is not by itself definitive evidence for d-wave pairing in bilayer nickelates.
- Evidence: comparison of simulated mirror-odd and mirror-even barrier conductance with conflicting reported point-contact spectra,consistency of nodeless ARPES/STM gaps with interlayer pairing
- The d-wave-like ZBCP should be tunable through layer asymmetry of the contact and disappear in the mirror-even limit of equal coupling to the two layers.
- Evidence: calculated contrast between mirror-odd (ZBCP) and mirror-even (gapped) barrier conductance
Workflow
- model_construction — A mirror-odd contact potential behaves as an effective Zeeman field on the layer pseudospin, binding in-gap Andreev states.
- Materials: minimal bilayer model with interlayer coupling and interlayer pairing; two-orbital tight-binding model for La3Ni2O7
- Methods: BdG Hamiltonian in Nambu basis with layer and particle-hole Pauli matrices; separation of contact potential into mirror-odd and mirror-even components; transformation to mirror (bonding/antibonding) eigenbasis
- Observations: mirror-odd potential acts as an effective Zeeman field on the layer pseudospin; pairing sign reversal moves into the choice of Andreev partners across layers
- bound_state_calculation — Interlayer pairing binds in-gap Andreev states whose energy depends on barrier width and strength.
- Materials: bilayer model with potential barrier on last sites
- Methods: analytic round-trip phase condition for Andreev bound states; SU(2) rotation treatment including interlayer coupling; numerical computation of bound-state energies versus barrier width and strength
- Observations: in-gap Andreev states sweep through the gap with barrier width and strength; bound-state energy can reach zero in several parameter regimes
- conductance_calculation — A mirror-odd barrier produces resonant Andreev reflection and a ZBCP, while a mirror-even barrier yields a gapped conductance.
- Materials: normal-metal–barrier–superconductor junction in the minimal model
- Methods: Blonder-Tinkham-Klapwijk formalism; multichannel BTK conductance formula; Lorentzian resonance analysis
- Observations: mirror-odd barrier produces a sharp zero-bias conductance peak at resonance; mirror-even barrier leaves conductance fully gapped; finite detuning shifts or splits resonances
- application_to_La3Ni2O7 — Simulated subgap conductance for a top-layer barrier reproduces the orientation-dependent ZBCP and side dips usually interpreted as a d-wave fingerprint.
- Materials: realistic two-orbital tight-binding model of La3Ni2O7; normal lead with same bands; barrier covering interface sites
- Methods: open-slab calculation of sub-gap Andreev states; multichannel BTK conductance evaluation; momentum-resolved coexistence-channel analysis
- Observations: broad ZBCP along one direction and zero-bias dip along the other; side dips below normal-state conductance; mirror-even barrier recovers conventional s-wave BTK line shape; ZBCP survives across a wide parameter regime
- interpretation — Interlayer pairing with a mirror-odd barrier reconciles nodeless bulk gaps with d-wave-like transport signatures in bilayer nickelates.
- Materials: existing point-contact, ARPES, and STM experimental reports
- Methods: comparison of simulated conductance with reported spectra
- Observations: nodeless bulk gaps from ARPES/STM are consistent with interlayer pairing; ZBCP and orientation dependence can arise without d-wave pairing